Scanning tunneling microscope used to encode one kilobyte at a density 500 times better than the best commercial hard drive

Every day, modern society creates more than a billion gigabytes of new data. To store all this data, it is increasingly important that each single bit occupies as little space as possible. A team of scientists at the Kavli Institute of Nanoscience at Delft University managed to bring this reduction to the ultimate limit: they built a memory of 1 kilobyte (8,000 bits), where each bit is represented by the position of one single chlorine atom. “In theory, this storage density would allow all books ever created by humans to be written on a single post stamp”, says lead-scientist Sander Otte. They reached a storage density of 500 Terabits per square inch (Tbpsi), 500 times better than the best commercial hard disk currently available. His team reports on this memory in Nature Nanotechnology on Monday July 18.

The team used a scanning tunneling microscope (STM), in which a sharp needle probes the atoms of a surface, one by one. With these probes scientists cannot only see the atoms but they can also use them to push the atoms around. “You could compare it to a sliding puzzle”, Otte explains. “Every bit consists of two positions on a surface of copper atoms, and one chlorine atom that we can slide back and forth between these two positions. If the chlorine atom is in the top position, there is a hole beneath it — we call this a 1. If the hole is in the top position and the chlorine atom is therefore on the bottom, then the bit is a 0.” Because the chlorine atoms are surrounded by other chlorine atoms, except near the holes, they keep each other in place. That is why this method with holes is much more stable than methods with loose atoms and more suitable for data storage.

STM scan (96 nm wide, 126 nm tall) of the 1 kB memory, written to a section of Feynman’s lecture There’s Plenty of Room at the Bottom (with text markup).

Codes
The researchers from Delft organized their memory in blocks of 8 bytes (64 bits). Each block has a marker, made of the same type of ‘holes’ as the raster of chlorine atoms. Inspired by the pixelated square barcodes (QR codes) often used to scan tickets for airplanes and concerts, these markers work like miniature QR codes that carry information about the precise location of the block on the copper layer. The code will also indicate if a block is damaged, for instance due to some local contaminant or an error in the surface. This allows the memory to be scaled up easily to very big sizes, even if the copper surface is not entirely perfect.

The advent of devices based on single dopants, such as the single-atom transistor the single-spin magnetometer and the single-atom memory has motivated the quest for strategies that permit the control of matter with atomic precision. Manipulation of individual atoms by low-temperature scanning tunnelling microscopy provides ways to store data in atoms, encoded either into their charge state magnetization state or lattice position. A clear challenge now is the controlled integration of these individual functional atoms into extended, scalable atomic circuits. Here, we present a robust digital atomic-scale memory of up to 1 kilobyte (8,000 bits) using an array of individual surface vacancies in a chlorine-terminated Cu(100) surface. The memory can be read and rewritten automatically by means of atomic-scale markers and offers an areal density of 502 terabits per square inch, outperforming state-of-the-art hard disk drives by three orders of magnitude. Furthermore, the chlorine vacancies are found to be stable at temperatures up to 77 K, offering the potential for expanding large-scale atomic assembly towards ambient conditions.

SOURCES – Delft University of Technology, Popular Mechanics, Nature Nanotechnology